[Electronica1]

You should be good with having two different size wheels in contact with the ground since you are using belt which should be able to handle it. As others have said, you don't need to lift the traction wheel up that far.

Also, have you thought about integrating the gearbox into the module? (see 488, 148, and 2826)

Have you had experience with 16 ft/s with mecanum? My team usually sticks around 14 ft/s with mecanum since it still gives you the power you need to strafe (mostly).